Composite support systems using plastics in combination with other materials

ABSTRACT

The invention describes a load-bearing system which is composed of a plurality of components of different materials. At least one component that dissipates load here is composed of plastic. A frictional bond achieved via at least one bonding technique transmits the loads for the various components. The plastic can be transparent.

A load-bearing system is described and is composed of a plurality of components of different materials. At least one component that dissipates load here is composed of plastic. A frictional bond achieves load transmission for the various components.

The load-bearing system described here utilizes the different strengths and specific properties of materials in order to obtain a load-bearing structure which is as slim as possible but nevertheless can withstand high loads, and which, depending on the nature of the plastics components, is in part transparent, translucent or opaque. The load-bearing system can be used either horizontally for example as a transverse-load-bearing element or else vertically as a prop.

Other load-bearing systems are also possible, examples being frameworks, Vierendeel trusses, arches, and also three-dimensional structures, such as sheets, plates, folded-plate structures or load-bearing shell structures.

PRIOR ART

Various composite load-bearing elements are known, some of these being transparent.

Timber-glass load-bearing elements: According to Prof. Julius Natterer and Dr. Klaus Kreher, a load-bearing element has been developed by combining timber and glass at the Ecole Polytechnique Federale de Lausanne, Switzerland. The load-bearing element is composed of a vertical glass sheet, with a frame composed of timber adhesive-bonded to both of its sides. The timber frame distributes the loads and provides tensile reinforcement for the glass sheet in the event that the sheet cracks when its flexural tensile strength has been exceeded. These timber-glass composite load-bearing elements have been used in the construction of a hotel in Switzerland.

(SOURCE: Dissertation by Klaus Kreher, EPFL Lausanne, 2002).

Concrete-glass load-bearing elements: Mr. Freytag has carried out experiments with a concrete-glass load-bearing element at the Technical University in Graz, Austria. Glass sheets, which have the function of dissipating stress loads, were combined with reinforced-concrete flanges.

(SOURCE: Dissertation by B. Freytag, Technical University of Graz, October 2002)

Timber I-beams: In 1969, Trus Joist was the first company in the world to produce an I-beam completely composed of timber. The load-bearing capability of the beams is provided by their constitution composed of laminated timber veneer as flange material and OSB as web material. The two fundamental materials are joined by a water-resistant glue, using heat and pressure. (SOURCE: Internet: http://www.trusjoist.com/GerSite/)

WO 2003/023162 describes a transparent structural element which has a sheet and which draws its load-bearing and stiffening properties from a frame surrounding all sides of the sheet. The sheet is a multilayer element composed of glass and/or polymer variants, these having been adhesive-bonded to one another. Various plastics are mentioned (claim 8 to 10), but only in a combination in a plurality of layers. Specifically, polycarbonate (PC), polyurethane (PU) and polyvinyl chloride (PVC) are used. There is no mention of polymethyl (meth)acrylates (PMMA) or of other transparent polymers, such as polystyrene (PS), acrylonitrile-butadiene-styrene copolymers (ABS), styrene-acrylonitrile copolymers (SAN), polyolefins, etc. Nor is there any mention of PMMA-glass laminates as material for the sheet of, and the web of, a load-bearing structure. The stiffening frame material was moreover described as a material composed of layers.

DISADVANTAGES OF THE PRIOR ART

When transparent load-bearing systems are considered, composite load-bearing elements involving glass are extremely fragile.

Glass is the stiffer material in a timber-glass load-bearing element and also in a concrete-glass load-bearing element. The consequence of this, on exposure to stress, is that the relatively brittle and flexurally stiff glass attracts the load. The stress within the glass is therefore higher than in the composite materials used. The glass is therefore also the first material to fail, and fails long before the materials in the combination can begin to exhibit their load-bearing capability.

Lack of transparency of a timber I-beam or other composite load-bearing elements prevents their use as a transparent design element with advantages in lighting and illumination, although their production is cost-effective.

OBJECT AND ACHIEVEMENT OF OBJECT

The object of the present invention consists in developing a composite load-bearing element in which plastics are used in accordance with the properties of these materials. When they are compared to the conventional construction materials, such as timber, steel, aluminium, glass, etc., they feature comparably low modulus of elasticity and high ductility. The supposed disadvantage of the low modulus of elasticity becomes an advantage in the appropriately devised composite with other materials. The combination leads to absorption of the high tensile and compressive stresses by the stronger materials and of the relatively small shear stresses by the softer materials. In contrast to other, known, load-bearing systems, the invention provides a frameless load-bearing structure which comprises a frictional bond between load-bearing parts of different materials. Plastics elements can be multilayer elements, or preferably single-layer elements composed of homogeneous materials. Another possibility is to develop load-bearing systems which are in part transparent, or coloured, or indeed luminous. Lighting elements that can be used are not only incandescent bulbs or fluorescent tubes but also LEDs. It is thus possible to meet almost any particular request relating to the optical properties of the load-bearing element. By virtue of the transparency of the plastics elements, the load-bearing structure is perceived as very filigree and lightweight. It is also possible to join props and transverse-load-bearing elements together to give a construction system.

The combination of plastics with other materials can produce a filigree load-bearing system. A load-bearing system here means a system involved in dissipation of load. It can either, like a transverse-load-bearing element or a cantilever, transmit loads in a horizontal direction, or, like a prop, transmit loads in a vertical direction.

In the case of a transverse-load-bearing element, the upper and lower part, here called flange, is composed of a stiff, conventional material, such as timber, steel, aluminium or glass, and the central part, here called web, is composed of one or more plastics. On exposure to load, by virtue of the marked difference in the stiffnesses, the conventional material attracts the loads, and the web serves merely to achieve equilibrium between the upper flange and lower flange. The two materials are bonded either via mechanical means of bonding, e.g. various screws or bolts, plugs, rivets, dowel pins, studs, etc., or by adhesive bonding. Other types of frictional bond are also conceivable here. The selection of the bonding technique is related to the manner of force transmission and therefore also to the load-bearing system under consideration.

In the case of a prop, the load-bearing system is composed, for example, of a plurality of small cross sections composed of conventional materials, prevented from buckling via bonding of the cross section with sheets of plastic.

Selection of Materials

Examples of stiffer material that can be used are conventional materials such as timber, timber materials, metals, glass or concrete, or high-performance plastics or reinforced plastics.

The less stiff material used can comprise plastics whose modulus of elasticity (measured to DIN EN ISO 527) is at least 150 N/mm², examples being poly(meth)acrylate (PMMA), polycarbonate (PC), acrylo-nitrile-butadiene-styrene copolymers (ABS), styrene-acrylonitrile copolymers (SAN), polyvinyl chloride (PVC) or polystyrene (PS). PMMA is marketed by Röhm GmbH with the trade mark Plexiglas®. For this use, Plexiglas® GS grades are particularly suitable, these being produced via cast polymerization. It is also possible to use filled grades of PMMA, these being marketed by way of example with the name Corian® or Creanit®. It is also possible to use laminates of different plastics or layered materials.

Production of Composite Load-Bearing Elements

In the production of the inventive article, means of bonding are used to secure the linear, conventional materials to a sheet-like plastics component. The sheet-like plastics component is very much longer in the direction of loading than perpendicularly to the direction of loading. The height:length ratio between two adjacent retention points of the component, also called bearing points, is by way of example from 1:1 to 1:80, preferably from 1:5 to 1:40 and very particularly preferably from 1:10 to 1:25. The height of the component is by way of example from 10 to 300 cm, preferably from 15 to 120 cm and very particularly preferably from 20 to 80 cm. The thickness of the component can by way of example be from 3 to 500 mm. The length of the component is selected as appropriate for the structural requirements, and the sheet-like plastics component can be converted to the required length via adhesive bonding. The linear, conventional materials are secured to the long edges. The bonding between the plastic and the conventional material is produced via means of bonding.

PRODUCTION EXAMPLE 1

Two commercially available slating battens whose cross section is 24*48 mm and whose length is 3 m are secured to each of the longer edges of a transparent sheet composed of Plexiglas® whose thickness is 10 mm and whose length is 3 m and whose width is 25 cm, with the aid of screw clamps, the battens therefore having opposite location on the sheet of plastic. Holes whose diameter is 8 mm are drilled at regular intervals (about 10 cm) through these two timber battens and the intervening plastics layer. Hexagon head cap screws are inserted through these holes and secured by a nut. The screw clamps are removed after assembly.

PRODUCTION EXAMPLE 2

An adhesive which solvates the material is applied to both sides along the longer edges of a transparent sheet composed of PMMA whose length is substantially greater than its width, an example being the adhesive marketed as Acrifix® by Röhm GmbH. Timber battens are pressed from both sides against the adhesion areas and fixed with screw clamps. After hardening and the resultant coherent bonding between plastic and timber, the screw clamps are removed.

Bonding of the Material (Means of Bonding)

The actual bonding of the material, between the individual elements that dissipate load, is particularly important, since this contributes decisively to the stability and load-bearing capability of the load-bearing structure.

A coherent permanent bond, where the materials involved in the bond are held together at the atomic or molecular levels is ideal. Familiar methods here would be adhesive bonding (welding, soldering) or vulcanization.

Frictional- or interlock-bonding techniques are a conceivable alternative method, and these also give sufficiently stable bonds. Clamping methods, and particularly screwing methods, may be mentioned here as a frictional bonding technique.

Possibilities of producing the load-bearing systems described by way of interlock bonding are provided via riveting, pinning (plugging), compression, shrinking, compressive jointing or thermoforming of the component materials. Some advantageous bonding techniques are described in detail below:

Partial combinations of the various bonding techniques are also conceivable.

Adhesive Bonding

Adhesive bonding should be the preferred coherent-bonding technique for using various materials to produce the load-bearing structure described here.

As a function of the selection of material, there is a wide variety of adhesive systems known in the literature which generally cover the following combinations:

metal-plastic, timber-plastic, metal-glass, metal-timber, etc.

According to DIN 16920, an adhesive is a non-metallic material which bonds adherends to one another by virtue of surface adhesion and internal strength. Suitable bonding adhesives therefore have to meet at least two requirements: they must produce sufficiently high adhesion to both the first and the second material, and they themselves must supply strength within the adhesive layer. Assessment of an “adhesive” bond of a material depends on the typical type of stress encountered in the application. In the present case of a composite load-bearing structure, the main stress present comprises shear, and rarely tension or indeed delamination. A sufficiently good criterion for assessment of adhesive bonds is therefore what is known as the shear strength, which involves separating the adherends from one another in a parallel direction. The more force required here, the better the bonding of the material.

Welding or soldering is mainly reserved for load-bearing structures composed of unitary materials, but can also be used as bonding technique in special cases, e.g. in metal-lightweight metal variants or plastic A-plastic B combinations.

Bonding Via Screwing Methods

Bonding via screwing methods can use almost any type of screwing method. This produces a frictional bond.

In the case of relatively soft materials, it is even possible to use self-tapping wood screws. If at least one hard material is involved at the bond, bonding of the materials involved has to be produced by way of the bearing surfaces within a hole, or screw threads on a screw or nut.

The force-transfer mechanism here in essence takes place in the bearing surfaces within the hole. The permissible stresses in the materials in the respective regions of the bearing surfaces within the hole cannot be exceeded, otherwise the material can break away or crack, thus weakening the load-bearing system. The selected size of the hole which has the bearing surfaces is generally slightly larger than the diameter of the screw. Appropriate screw-fixing methods have to be selected as a function of the use of the load-bearing system.

Pegged Bonds

Pegs used here comprise either timber pegs or else any other types of peg, such as steel pins, or springs. These pegs are intended to produce a bond in pre-drilled holes.

Bonding Via Thermoforming

Bonding can also be achieved between the plastic and the other material via thermoforming. Here, a heated thermoplastic material inserts itself into an irregular groove in the conventional material. The irregularity in the groove produces cavities into which the thermoplastic material inserts itself and therefore “grips”.

Bonding Via Shrinkage

Cooling is used to bring, for example, the plastics part to a very low temperature. This causes shrinkage of this plastics part. The plastics part is now introduced with precise fit between two components composed of conventional material. Heating the plastics component to normal temperature causes it to expand and thus become clamped between the conventional material.

Means of bonding: screws or bolts, studs, pegs, adhesives, rivets, dowel pins, sintering, or any of the known mechanical and adhesive bonding techniques.

EXAMPLES Timber-PMMA I-Profile

One possible example of the use of the load-bearing system described in the construction industry is a transverse-load-bearing element with an I cross section composed of various materials. The upper and lower flange of the load-bearing element is composed here of a traditional construction material, e.g. metal or timber, while the web is produced from a plastic. The web ideally has lower stiffness than the two flanges, since this ensures that the majority of the normal stresses occur in the flanges. The plastics web transfers the shear forces between the two flanges. The two different materials are bonded with means of bonding in the shape of pegs. Examples of means that can be used here are studs or plugs. Appropriate adhesive bonding would also be possible. A transparent plastic, such as PMMA, gives the load-bearing element low perceived weight, which is of high aesthetic value.

The height of the load-bearing element varies from 10 to 300 cm, the thickness of the plastics webs being from 3 to 500 mm. The cross-sectional area of the flanges is in the range from 5 to 3000 cm² in the case of timber, and from 1 to 500 cm² in the case of steel.

In the example constructed (see drawing No. 1), a load-bearing element of height 25 cm was constructed with a Plexiglas® XT 20070 PMMA sheet of thickness 10 mm. The flange material used in each case comprised two commercially available slating battens of dimensions 24*48 mm. The means of bonding used comprised screws whose diameter was 8 mm with about 10 cm separation. A deflection of about 2 cm was measured on exposure to a load of 5000 kg (as shown in drawing 6).

Underbraced Load-Bearing Element

Another possibility for a load-bearing system composed of transparent plastics in conjunction with known types of structure is an underbraced load-bearing element composed of a known material, for example aluminium or timber, of a plastic and of a bracing cable. The load-bearing element has an upper flange which accepts the compressive forces and a possibly transparent plastics web whose underside has a milled groove which serves as guide for a cable. The load-bearing element has a fish-belly shape, thus permitting the cable to be connected at the end of the load-bearing element with the pressure flange. Both the upper flange and the underside of the load-bearing element here can have a curved shaped.

Solid Load-Bearing Element

The system described here can also be applied to a solid beam. Here, two lamellae of a conventional construction material are adhesive-bonded, or fixed with mechanical means of bonding, to the upper and lower side of a solid plastics beam for reinforcement. In this case, too, the respective flange is again mainly responsible for the normal stresses. In selecting materials here, glass may also be mentioned as flange material, since this can give a completely translucent load-bearing element in the case of combination with a transparent plastic. Another conceivable variation is a solid plastics beam with a filament composed of steel on the upper and lower edge of the load-bearing element. This steel filament accepts the tensile forces and therefore provides a type of reinforcement for the plastics load-bearing element in a manner similar to that in reinforced concrete.

Prop

Bonding of conventional materials to a plurality of transparent plastics sheets can give a prop which is perceived as extremely slim. In this multi-part member, intended for compression, the compressive forces are accepted by, for example, four metal rods, while the sheets of plastic stabilize the individual compression rods and thus prevent buckling. The moment of inertia of the prop is more important here than the cross-sectional area, and a non-solid cross section therefore provides an alternative which is markedly more filigree and lightweight and moreover saves material. In plan view, there are many possible variants and shapes for the arrangement of the individual compression members and sheets, but for reasons of static efficiency the location of the metal elements or timber elements should be at maximum distance from the centre of gravity.

Key:

-   1: conventional material -   2: plastic -   3: means of bonding -   4: steel filament -   5: weight (1000 kg) 

1. A composite load-bearing system, wherein plastics are joined with at least one further material to give a load-bearing system.
 2. The composite system according to claim 1, wherein the materials used have different moduli of elasticity.
 3. The composite system according to claim 1, wherein the modulus of elasticity of the plastics material is more than 150 N/mm².
 4. The composite system according to claim 1, wherein the stiffer material is further from the centre-of-gravity axis.
 5. The composite system according to claim 1, wherein the softer material is a transparent plastic.
 6. The composite system according to claim 1, wherein the softer material is a translucent plastic.
 7. The composite system according to claim 1, wherein the softer material is a coloured plastic.
 8. The composite system according to claim 1, wherein the softer material is a light-emitting plastic.
 9. The composite system according to claim 1, wherein there are cavities in the softer material.
 10. The composite system according to claim 1, wherein the softer material can be a plastics laminate.
 11. The composite system according to claim 1, wherein the softer material can be a material composed of layers.
 12. The composite system according to claim 1, wherein the harder material is timber or a timber material.
 13. The composite system according to claim 1, wherein the harder material is a metallic material (iron, steel, aluminium) selected from the group consisting of iron, steel and aluminum.
 14. The composite system according to claim 1, wherein the harder material is glass.
 15. The composite system according to claim 1, wherein the harder material is concrete or a natural stone.
 16. The composite system according to claim 1, wherein the harder material is a filled or glass-fibre-reinforced plastic.
 17. The composite system according to claim 1, wherein the selected means of bonding comprise screws or bolts.
 18. The composite system according to claim 1, wherein the selected means of bonding comprise pegs or dowel pins.
 19. The composite system according to claim 1, wherein the selected means of bonding comprise studs.
 20. The composite system according to claim 1, wherein the selected means of bonding comprise rivets.
 21. The composite system according to claim 1, wherein the selected means of bonding comprise an adhesive.
 22. The composite system according to claim 1, wherein the bonding is based on friction.
 23. The composite system according to claim 1, wherein the bonding has been is produced with the aid of thermoforming.
 24. The composite system according to claim 1, wherein two or more means of bonding are used in combination.
 25. The composite system according to claim 1, wherein it is loaded as a transverse-load-bearing element, horizontally.
 26. The composite system according to claim 1, wherein it is loaded as a prop, vertically.
 27. The composite system according to claim 1, wherein the two materials are involved in the structure in the shape of an I.
 28. The composite system according to claim 1, wherein the materials are bonded to give a solid load-bearing element.
 29. The composite system according to claim 1, wherein, when viewed in cross section, it has the shape of a polygonal box.
 30. The composite system according to claim 1, wherein, when viewed in cross section, it has the shape of a folded-plate structure.
 31. The composite system according to claim 1, wherein it has been underbraced with a stiffer material.
 32. The composite system according to claim 1, wherein a combination of elements is used. 